1. Field of the Invention
This invention relates to modem technology, and in particular maintaining connection during loss of controller synchronism.
2. Background
Modems typically connect two computers across telephone lines. Current telephone lines are designed to carry voice signals in the form of a modulated analog wave form. Modems convert digital data from a computer into an audio wave form that can be sent over current telephone lines. A first computer system, designated calling computer, instructs a modem to dial and establish a connection with a second modem. The second modem is connected to another computer system, designated receiving computer. The first modem converts digital information from the calling computer into an audio wave form to send across telephone lines. The second modem receives the audio wave form from the telephone lines and converts the wave forms into digital information which is sent to the receiving computer. This communication link can be bi-directional. In other words, both computer systems may send and receive information through the modems to the other computer system.
The modems communicate to each other via communication protocols. These communication protocols include modulation protocols, error control protocols and data compression protocols. Modulation protocols define the specific techniques of encoding and decoding the digital bits into the audio wave form and the data transfer speed. Two modems can establish a connection only when they share a common modulation protocol.
So that modems from different manufactures can communicate, there are several industry established communication protocols. Two standard modulation protocols for high speed modems are V.32 and V.32bis, established by the CCITT (the International Telegraph and Telephone Consultative Committee). V.42, established by CCITT, is an example of an error control protocol. V.42bis, established by CCITT, is an example of a data compression protocol.
To establish a connection between two modems, a training signal is typically used. This involves establishing a reference signal in the form of an audio wave form between the two modems in order to synchronize the interfaces. Once synchronization is established, the modems can send and receive data. All subsequent audio wave forms received are compared to the reference signal in the decoding of the digital information. A training signal occasionally needs to be reconfigured after a connection is already established. This occurs to recover from various disruptions such as line outages, bursts of noise on the line, or other such line interference.
The conversion of the digital data from a computer into an audio wave form by the sending modem is accomplished by using the reference wave form. By varying the amplitude and phase of the audio wave form compared to the reference wave form, digital data can be encoded. Different states are assigned to different bits. Amplitude is the loudness of the signal. There may be two or more states for amplitude, such as states loud and soft. Phase refers to the phase angle difference of the audio wave form when compared to the reference wave form. Adding phase states allows more data bits to be encoded. For example, with two amplitudes, and four phases, three bits of data can be encoded. Data bits 000 can be defined as soft, zero degree phase, data bits 001 can be defined as soft, 90 degree phase, data bits 111 as loud, 270 degree phase, etc. The addition of more amplitude and phase states allows additional data to be encoded.
The sending modem converts digital data from the computer system and sends the information across phone lines to another computer system. Occasionally, the computer system fails to send enough data to the modem to keep the line active.
When not enough data is received by the sending modem, the modem typically runs out of data to send and the connection may be lost. A variety of conditions may cause the computer system to fail to send enough data to the modem. For example, the computer may be too busy doing other tasks. Some applications bog down a computer system when large amounts of data need to be transferred causing the bus to exceed bandwidth limitations.
Loss of the communication link can be annoying as well as expensive to a computer user. The connection must be reestablished, transferred data may be lost and must be resent, reconnection costs are incurred including additional toll charges and on-line service charges, and time is lost.